The invention relates to a metering system for introducing a reducing agent into an exhaust-gas chamber of a combustion unit without using compressed air.
Catalytic cleaning processes can be used to remove pollutants from an exhaust gas of a combustion unit. Catalytic cleaning processes in which a pollutant, as a first reactant, reacts with a medium that is additionally introduced into the exhaust gas, as a second reactant, are used in exhaust-cleaning units that are operated using a reducing agent.
In exhaust-cleaning units that are operated using a reducing agent, the medium that is to be introduced into the exhaust-gas chamber is a reducing agent that reduces one or more pollutants in the exhaust gas. The amount of the reducing agent to be introduced into the exhaust-gas chamber depends on the amount of pollutant in question. The amount may vary at different times. The amount of reducing agent to be introduced at any one time is usually set through an electrical control unit.
If the reducing agents used are dissolved solids, in particular urea (NH2xe2x80x94COxe2x80x94NH2), heating of the reducing agent may lead to evaporation and/or flocculation of individual solution constituents. Consequently, solids are left behind. Once the solids have been heated up accordingly on the parts that are in contact with the exhaust gas, they can generally no longer be removed. Residues of this nature may lead to operating disruptions in the pipe system carrying the reducing agent. For example, there is a risk that a nozzle generally used to introduce the reducing agent into the exhaust-gas chamber becomes mechanically blocked. Therefore, the heat of the exhaust-gas chamber, in which temperatures of, for example, up to 600xc2x0 C. prevail, should not heat the reducing agent (which is at a temperature of, for example, ambient temperature) before it is introduced into the exhaust-gas chamber.
To prevent heating of the reducing agent, cooling compressed air generally assists with injecting the reducing agent into the exhaust-gas chamber of the combustion unit. For such purpose, European Patent 0 558 452 B1 discloses carrying out the injection with a two-fluid nozzle and, in this way, passing the required compressed air along the feed line as a coolant. The flowing compressed air also cools the nozzle that opens out into the exhaust-gas chamber. Alternatively, International PCT publication WO 96/36767 discloses mixing air and reducing agent in a premix chamber. The premix chamber is connected to the exhaust pipe through a line and a nozzle such that the hot exhaust gas cannot heat the premix chamber. Moreover, the added air prevents the reducing agent from being heated.
However, assisting the metered introduction of reducing agent into an exhaust-gas chamber of a combustion unit with compressed air requires considerable technical outlay. Moreover, during operation of such a combustion unit, in addition to controlling the metered introduction of the reducing agent into the exhaust-gas chamber, it is additionally necessary to constantly monitor a technically complex compressed-air system.
It is accordingly an object of the invention to provide a metering system that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that introduces a reducing agent into an exhaust-gas chamber of a combustion unit with particularly little technical outlay and operates at a particularly low monitoring level. The invention provides these characteristics by providing an electrically controllable valve, the outlet opening of which is disposed in the exhaust-gas chamber.
With the foregoing and other objects in view, in a combustion unit having an exhaust-gas chamber and a reducing agent reservoir there is provided, in accordance with the invention, a metering system, including an electrically controllable valve for introducing reducing agent into the exhaust-gas chamber without using compressed air. The valve has an outlet opening disposed in the exhaust-gas chamber, and the outlet opening is fluidically connected to the reservoir through the valve.
The invention is based on the consideration that a metering system requiring a particularly low level of monitoring in operation can be made with little technical outlay if it is possible to dispense with a technically complex compressed-air system. A technically complex compressed-air system can be dispensed with if the region between the electrically controllable blocking point in the reducing-agent line and the feed-in point of the reducing agent into the exhaust-gas chamber (which region has to be cooled by compressed air) is made particularly small. For such purpose, the outlet opening of the electrically controllable valve opens out directly into the exhaust-gas chamber.
In accordance with another feature of the invention, in order to prevent heating of the reducing agent in the regions that can be heated by the heat of the exhaust gas flowing through the exhaust-gas chamber in a particularly reliable way, a recirculating circuit for the reducing agent is advantageously provided. A holding chamber for the reducing agent, which is disposed directly in front of the valve outlet opening (downstream of the outlet opening in a reducing agent flow direction), is expediently connected into the recirculating circuit. Therefore, the reducing agent that is held in the holding chamber can be kept at a constantly low temperature, despite its proximity to the exhaust-gas chamber, because the agent is constantly recirculated. As such, there is no risk of the reducing agent evaporating and/or flocculating.
In accordance with a further feature of the invention, a coolant circuit can advantageously cool the valve. The coolant circuit may be integrated into the cooling system of the combustion unit in question. The valve may, for example, be situated in an adapter through which coolant flows. With such a configuration, the cooled valve and/or the recirculating circuit ensure(s) additional cooling of the reducing agent.
In order to bring about particularly good mixing of the reducing agent with the exhaust gas that is being carried in the exhaust-gas chamber of the combustion unit, the valve is advantageously disposed on the exhaust-gas chamber such that its principal axis includes an angle of less than 90xc2x0 with the principal direction of flow of the exhaust gas in the exhaust-gas chamber. As a result, the reducing agent is entrained, in a particularly simple manner, by the exhaust gas when the agent enters the exhaust-gas chamber, thus promoting mixing.
Advantageously, the metering system also has a baffle that is disposed in the exhaust-gas chamber. The baffle is disposed such that it causes a change in direction of the reducing agent that has been introduced into the exhaust-gas chamber. The associated additional dispersion of the reducing agent in the exhaust-gas chamber is expedient, in particular, in order to bring about particularly complete reaction of the reducing agent with a number of pollutants contained in the exhaust gas.
In accordance with a concomitant feature of the invention, the metering system is advantageously used for introducing urea solution into an exhaust-gas chamber of a diesel engine. The advantages are provided because urea solution has been found, in particular, to be especially sensitive to heating. For example, such heating is imparted from the heat of the exhaust gas flowing through the exhaust-gas chamber because, in the event of individual solvent constituents of the urea solution evaporating, solids that cause operating disruptions to the metering systems may be left behind.
The introduction of urea solution is used, in particular, in the selective catalytic reduction (xe2x80x9cSCRxe2x80x9d) process, as a precursor to ammonia (NH3), which does not have a neutral odor. The SCR process is usually employed to convert nitrogen oxides (NOx) to form nitrogen (N2) and water (H2O) with the aid of ammonia (NH3) and a catalyst.
The advantages achieved with the invention include, in particular, the fact that the metering system is implemented and operated particularly simply by dispensing with a technically complex compressed-air system. It is possible to dispense with such a system by using an electrically controllable valve having an outlet opening directly into the exhaust-gas chamber. Such a configuration is associated with a particularly low technical outlay because the electrically controllable blocking point in the reducing-agent line then substantially coincides with a point where the reducing agent enters. The valve used may be a standard fuel injection valve.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a metering system, it is, nevertheless, not intended to be limited to the details shown since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.